I have yet to hear about a rabbit breaking it's back because of a harness. You just need to make sure it's fastened properly.
Mine jump around even when wearing one, as long as the lead is long enough that's not a problem. I sometimes use a dog-lead that can be pulled out to three meters (though I'm planning to get one on 5 meters, since the handle on my old one is too small)
There are various types of harnesses, some are straight out dangerous which is why NKHF, the society for show jumping in Norway, have strict rules on the harnesses and leads, to make sure no rabbit is injured while wearing one.
Most people I know who has rabbits use harnesses, plenty of them do show jumping, and I have never heard of a rabbit getting injured in this competitions, despite the fact that they all jump with harnesses on, sometimes also very high